America's Cup boss Russell Coutts justified the reduction in the size of the boats after the last edition, which was still sailed on 72-foot monsters, with a reduction in costs. First it was supposed to be 62-foot cats, now it has become 50 feet. Thanks to a variety of further developments, these new racing boats are significantly faster than their predecessors, reaching speeds of up to 50 knots.
However, it hardly got any cheaper for the teams, as the budgets of up to six-figure millions show. Which is surprising. In addition to reducing the size of the boats and the sailing crews, it was also decided to standardise many components. For example, the profile of the wing and the front wing section, the shape and design of the hulls, beams, the centre section (pod) and the bowsprit are standardised. This was also ostensibly a cost-saving measure.
How the Cup cars learnt to fly
In fact, however, some suspect that this is due to a competitive advantage secured by defenders Oracle Team USA. The team of software billionaire Larry Ellison was caught cold at the last Cup when the New Zealanders made their boat fly. This was not supposed to be possible due to the rule that still applies that only one centreboard can be in the water at any one time. The New Zealanders found a way and the Americans only managed to avoid defeat with great effort.
The defender obviously didn't want to experience such a surprise again this time. "A rich team always has an interest in restricting the rules as much as possible," says Martin Fischer. The German foil expert is the chief designer of the French team Groupama. "If you have more or less the same as everyone else," Fischer continues, "if you have more money, you can train more, refine more and will win in the end. With an open rule, on the other hand, there is a risk that someone will find something that you haven't thought of yourself." So although the designers' playing field has been reduced, the amount of detailed work has been increased.
The Foils will win the Cup
The design of the foils and their adjustability is something like the holy grail of this cup. It is no longer an art to get a catamaran to fly, i.e. to hoist it on foils. The task in the America's Cup is rather to achieve this as early as possible, i.e. in very light winds, and to keep the cat in the air for the entire race. The hydrodynamic resistance should always be as low as possible.
Even a single touchdown, i.e. touching the surface of the water with the hulls, can cost crucial metres. It is important to weigh up the ratio of forces. The rule is: the more buoyancy, the greater the resistance. And the more aggressive the shape, the lower the resistance, but the more unstable the flying position.
The AC 50 will fly on the entire course
In the previous edition, the centreboards were still more in the V position. This guarantees the most stable flight attitude and regulates the flight altitude quite well. This position is also used for beach cats or the GC32 catamaran.
Today, however, the centreboard tends to be ridden vertically on all courses, upwind and downwind. This minimises drift, which is also important downwind, but above all minimises drag. This vertical position is possible due to the highly developed operation of the foils' angle of attack (more on this in a later article). The centreboard can be adjusted extremely sensitively by the helmsman, who can react immediately to any change in height. A self-adjusting position is therefore no longer necessary.
Not only can the helmsman react, he has to. This is because the foils used are permanently unstable in order to minimise drag. The foils are also three-dimensionally adjustable, which opens up enormous possibilities. Unprecedented graphics of a Land Rover BAR prototype show just how complicated the adjustment is.
Each team is allowed to use four foils during the Cup. How these are designed also depends on the race tactics. The simplest would be two sets of foils: one for light wind with more buoyancy, one for high wind. However, there could also be four different foils, and the boat could take to the race course asymmetrically (more on this in the later article "Race tactics").
The foils are either laminated with a sandwich core, which makes them lighter but also deforms them more, or made from full laminate. Depending on the type of production, such a foil costs between 200,000 and 400,000 euros and takes up to twelve weeks to build.
During the race, an AC 50 sails on three "legs": the leeward centreboard and both rudders. Only one centreboard may be in the water at any one time, except when tacking or gybing. Then both may be in the water for a maximum of 15 seconds.
The leeward centreboard is used to regulate lift and drift. The leeward rudder also generates lift and a bow-heavy trim. The windward rudder, on the other hand, is set to downforce. It pulls the hull downwind and thus provides additional righting moment.
The angle of attack of the rudders must change when tacking and gybing, as otherwise they would act the wrong way round on the new bow. This is done automatically. When the manoeuvre is initiated, the helmsman gives the appropriate command and the rudders change their angle of attack. Once this command has been given, there is no turning back or the cat will flop onto the water.
Recently, more foils with a slight S-shape in the vertical section have been seen. This serves to increase the righting moment. When the centreboard is lowered, the base of the tip, the horizontal part, moves outwards until it reaches the maximum hull width. If the centreboard shaft were exactly straight, the section of width between the centreboard box and the outside of the fuselage would be lost for the righting moment.
The big Cup guide: In the America's Cup special in YACHT 12/2017 you will find all the information and background to the sailing event of the year. From 24 May at the kiosk or digital here.
Lars Bolle
Chief Editor Digital
